Method for bonding a flexible cable to an electrical component

ABSTRACT

A method for bonding the ends of the conductors 52 to circuit pads 54 is disclosed whereby the cable 20 is placed over the circuit pads 54; a bonding tool 44 is pressed against an upper surface 58 of the cable 20; the substrate 46 of the cable 20 is compressed beyond its elastic limit; and ultrasonic energy is applied to the bonding tool 44 and transmitted through the substrate 46 to effect a metallic bond between the conductors 52 and circuit pads 54.

This is a division of application Ser. No. 394,049, filed Aug. 14, 1989,now U.S. Pat. No. 5,183,973, issued Feb. 2, 1993.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to an apparatus for interconnecting the detectorassembly of an infrared system to the processor electronics of thesystem, and to a method for connecting the apparatus to the circuit padsof an infrared detector. More generally, the invention relates to anapparatus for interconnecting electronic components.

2. Discussion

A significant number of infrared detector/dewar applications requireoperation at cryogenic temperatures (<135° K). In order to minimize thethermal loads on the cooling devices of such systems, the detectorassembly is typically packaged within a vacuum and mounted directly ontothe coldfinger within a dewar. A thin film flexible cable is then usedto electrically interconnect the cooled detector assembly with anambient temperature input/output connector, thereby providing means forelectrically connecting the cooled detector with external, ambienttemperature processor electronics. Because the cable acts as a thermalshort between the coldfinger and the ambient environment, thermalperformance is the primary consideration in the design of the cable.

The two thermal characteristics of a cable that most heavily affect itsthermal performance are the thermal conductance and I² R heating of theconductors of the cable. Of these two, thermal conductance is usuallythe most critical. To minimize thermal conductance the conductormaterial is selected based on its electrical and thermal conductivityproperties. Typical conductor materials like gold and copper exhibit theoptimum electrical conductivity characteristics; however, the smallcross-sectional area of the conductor, which is necessary so as not toexceed the thermal conductance requirements, is far below industryprocessing limits. Consequently, constantan, nickel or stainless steelhave become the metal-foils of choice because of their low thermalconductivity properties. But even with these metals, the conductorlengths required to achieve a sufficiently large thermal resistance aremany times longer than that required to interconnect the detectorassembly and the input/output connector.

It would be desirable to have a thin film cable whose conductors aremade of gold. Such a cable would provide optimum thermal performance inconnecting the cooled detector assembly of an infrared array with theambient temperature input/output connector of the dewar.

An additional problem has been inadequate bond or adhesive strengthbetween the metal-foil conductors and the dielectric substrates adheredto each side of the conductors. This problem has been particularly,acute with respect to thin film cables used to connect to an infrareddetector assembly, operating at cryogenic temperatures.

Other disadvantages exist. Prior art metal-foil conductors, beinglaminated to dielectric substrates on each side of the metal-foil,generally require the use of material that can contaminate the dewarenvironment resulting in reduced vacuum life and lengthy pump-downtimes. Metal-foil cables also require complex multi-operationmanufacturing processes that are low yield and result in a high unitcost. In addition, laminated cables are not very flexible and continuedworking or bending of them during the assembly process often results inseparation of the foil and dielectrics. Moreover, the non-flexiblenature of laminated metal-foil cables typically requires that each endof the cable be attached to a part of the dewar structure to provide astrain relief for the fine wire interconnections and to protect themfrom damage during thermocycling of the dewar. It would therefore beadvantageous to have a thin film cable of simple construction thatavoids the use of adhesives and the like in the manufacturing process,which is more flexible than a laminated metal-foil cable without itsconductive elements being prone to separation from its substratematerial, and which operates reliably in a cryogenic environment.

There are two major prior art methods of connecting thin film cables tothe circuit pads of infrared detectors and other electronic components.The first consists of attaching a metal-foil cable to a part of amounting structure at each end of the cable with adhesives or mechanicalclamps and interconnecting the cable to the circuit pads of a detectoror other electronic component by wirebonding fine wire to the circuitpads and the conductors of the cable. The second uses a direct fine wireconnection from a circuit pad to a second connection point.

A problem exists in both prior art methods in that completion of theinterconnection is dependent on fine wire interconnections achieved bywire bonding. These wirebonds are frail, easily broken, and require twojoints in a circuit line (one at either end of the wire).

A third prior art method of connection, Tape Automated Bonding (TAB),eliminates the need for wirebonding by direct bonding of the conductorto the circuit pads of the detector or other electronic device. However,with TAB, substrate material is usually removed around the conductor bytoxic agents or the like so that a bonding tool can make actual contactwith the conductor material. This extra step is costly and inconvenient.

It would therefore also be desirable to have a method of connecting athin, film cable to the connector pads of an infrared detector or otherelectronic device that eliminates the need for the fine wire connectionsassociated with wirebonding or the need to remove a portion of thesubstrate material required by the TAB connection method.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations associated with priorart cables and bonding methods by providing a unique thin film cable anda unique method for attaching the cable to circuit pads of an infrareddetector or any other electronic device having circuit pads.

The thin film cable comprises a substantially flat, thin insulativesubstrate having an upper and a lower side; a layer of nonorganicadhesive material adhered to the lower side of the substrate; and aplurality of gold conductors adhered on the adhesive material.

The thin film cable is attached by placing it over a plurality ofconnection points so that the ends of the conductors lie on theconnection points; pressing the upper side of the substrate over ends ofthe conductors with an ultrasonic bonding tool having a pair oforthogonal grooves of unequal size in its tip; and applying ultrasonicenergy to the tool and through the substrate while applying downwardpressure for a sufficient period of time to bond the conductors to theconnection points.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present Invention will become apparent toone skilled in the art by reading the following specification and byreferencing the following drawings in which:

FIG. 1 is a fragmentary perspective view of an overall infrareddetector/dewar system using the flexible thin film cable of the presentinvention;

FIG. 2 is a perspective view of a typically shaped thin film cableaccording to the present invention bonded to the circuit pads of aninfrared detector and the pins of an input/output connector;

FIG. 3 is an illustration of a bonding tool in position to bond a firstend of the thin film cable to the circuit pads of the infrared detector;

FIG. 4 is an enlarged side view of the tip of the bonding tool;

FIG. 5 is an enlarged side view of the tip of the bonding tool similarto FIG. 4 except that the tip has been rotated 90°; and

FIG. 6 is a sectional view taken along the lines 6--6 of FIG. 4 showingmore fully the geometric relationship between the two grooves in the tipof the bonding tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 there is illustrated an overall infrareddetector/dewar system 10 in which the present invention may be used. Aninfrared detector 12 is disposed on an interior base portion 14 of acoldshield 16 defining an Interior area 18 which is cooled down tocryogenic temperatures. The flexible cable 20 is shown having a firstend 22 electrically connected to the detector 12 and a second end 24connected to pins 26 of a hermetically sealed Input/output connector 28.The pins 26 run vertically through the connector 28 and provide a meansfor electrically connecting the thin film cable 20 with externalprocessor electronics (not shown).

A vacuum housing 30 is further shown for maintaining a vacuum in area 32at an ambient temperature. The system 10 further includes a tubularportion 34 having an apertured coldfinger portion 36 therethrough, agetter 38 for removing moisture and residual gasses from the internalconfines of the system 10, and a mounting flange 40 attached to thetubular portion 34.

In FIG. 2 an enlarged perspective view of a preferred embodiment of thethin film cable 20 is shown. As can be seen more clearly, the first end22 of the thin film cable 20 Is placed in contact over a portion of thedetector 12. The second end 24 is placed in contact with the connectorpins 26 of the input/output connector 28. In practice, the thin filmcable 20 may be configured in any desired shape that provides thedesired thermal and electrical characteristics for a specificapplication. In FIG. 2, however, a rectangularly-shaped structure forthe thin film cable 20 is shown. A rectangular-shape is preferable whenthe thin film cable 20 is used in dewar applications, as such a shapehelps to provide the desired thermal and electrical characteristics formatching the cooled detector with the external, ambient temperatureprocessor electronics, while fitting properly within the confines of thevacuum housing 30 shown in FIG. 1. It should be appreciated, however,that while the thin film cable 20 is particularly well-suited toInfrared detector/dewar applications, its ability to provide specificthermal and electrical loading characteristics will make it valuable ina wide variety of electronic applications.

In FIG. 3 there is an enlarged view further illustrating the substrate46 or the thin film cable 20 In contact with a conically-shaped tip 42of an ultrasonic bonding tool 44. The substrate 46 of the thin filmcable 20 is a thin polyimide film having a thickness in the range of0.0003 to 0.020 inches, preferably between 0.0005 to 0.0030 inches, andmore preferably 0.001 inches. It is presently preferred to usepolyimide-H film, although other types of film may be suitable.

The substrate 46 further has a layer of titanium 48, preferably in therange of 400 to about 2,000 angstroms thick, sputtered uniformly ontoits underside 50 for strengthening the bond of a plurality of conductors52 (of which only one is visible In FIG. 3) to the underside of the thinfilm cable 20. Although titanium Is the preferred material forstrengthening the bond of the conductors, other nonorganic metallicmaterials such as tungsten or molybdenum followed by chromium, such asis used in semiconductor fabrication may also be used.

Prior to sputtering the layer of titanium 48 onto the polyimide film 46,the film 48 is first secured by stretching it radially over a ring andfastening it thereto in a manner similar to the operation of embroideryhoops. It is presently preferred to use such rings having diametersbetween three and four inches. The polyimide film 46 is then cleanedwith methanol to remove oils. Thereafter, oxygen plasma cleaning isused, and the surface of the polyimide film 46 is then limited toexposure to the atmosphere and the moving charges therein to the extentpossible. Prior to the deposition of a layer of titanium 48 bysputtering, the polyimide film 48 is reverse sputtered or etched in avacuum deposition chamber using argon in order to remove oxides and toallow the adhesion of metal films to the polyimide film 46. Presently,it is preferred to remove approximately 10 to 20 angstroms of materialfrom the surface of the polyimide film. This can be done using a thinfilm sputtering system sold by Material Research Corporation operatingat a pressure of approximately 3.5 millitorrs at 400 volts for tenminutes. Thereafter, the polyimide film is exposed under a titaniumtarget in a sputtering system using argon ions at a pressure of 7millitorrs and a voltage of 2 kilovolts for ten minutes. While athickness of approximately 1,500 angstroms for the layer of titanium 48is presently preferred using this system, it should be realized thatlayers of other thicknesses may be used. If the titanium is too thin, itmay be inadequate in view of subsequent chemical processing steps. Onthe other hand, differences in thermal expansions of adjacent materialsand other causes may lead to failure in the case where the titaniumlayer is too thick. In particular, if the layer is too thick,delamination may result. It will be appreciated by those skilled in theart that the actual limits for the thickness of the titanium layer 48will vary depending upon circumstances such as the thickness of thepolyimide film 46, the ultimate electrical configuration desired andalso on the particular equipment used. For example, using a magnetronsputtering system sold by Consolidated Vacuum Corporation, it ispresently preferred to have a titanium layer about 600 angstroms thick.It is presently preferred to have the thickness of the titanium layer inthe range from about 400 to 2,000 angstroms.

To form the conductors 52, gold is sputtered onto the titanium layer 48.Presently, it is preferred to perform the gold sputtering in the samesputtering system as that in which the titanium layer 48 is deposited,so that the titanium layer does not become oxidized. This may beaccomplished by moving the polyimide film 46 having the titanium layer48 under a gold target and operating the Material Research Corporationsystem at a pressure of 5 millitorrs and at a voltage of 2.5 kilovolts.Because of the high molecular weight of gold, distortion of thepolyimide film during the sputtering process is avoided by limiting thethickness of the sputtered gold conductors 52. The gold conductors 52will preferably be sputtered to a thickness of about 1,000 to 3,000angstroms, more preferably 1,700 angstroms. The gold conductors 52should be sputtered to a thickness at least sufficient to preventoxidation of the titanium layer. A layer of 500 angstroms or less maysuffice in some cases, while 1,700 angstroms would usually besufficient. Using the Consolidated Vacuum Corporation System, forexample, a layer of 1,000 angstroms is presently preferred.

Additional gold may then be electroplated onto the conductors 52 toachieve a predetermined total thickness needed to meet specificelectrical and thermal requirements. When connecting the thin film cable20 to a detector operating in a cryogenic environment, theserequirements will usually mandate the electroplating of additional goldonto the conductors 52 to achieve a total thickness in the range of25,000 to 100,000 angstroms. The gold conductors 52 are then etched to awidth in the neighborhood of 0.0002 to 0.0005 inches, depending again onthe desired performance characteristics, to provide a distance betweenthe centers of adjacent conductors of 0.0004 to 0.010 inches. This maybe accomplished using photoresist to obtain the separate conductors 52.It is presently preferred to use a high viscosity photoresist that doesnot bead easily and that may be applied in a thick layer that is softlybaked before exposure through a mask and subsequent removal of gold andtitanium to delineate the conductors 52. Alternately, separateconductors may be delineated before the electroplating of gold onto thedeposited gold occurs.

In addition, the ends of the conductors may be plated-up with additionalgold to achieve the necessary thickness for the attachment bond, whichwill require a total thickness of approximately 50,000 angstoms toachieve a proper metal to metal bond. For simplicity, however, theconductor 52 shown in FIG. 3 is illustrated having a uniform thicknessthroughout its length.

The particular voltages, pressures and times used to perform the varioussteps in fabricating the metal adhesive and gold layers on the polyimidefilm will vary depending on the specific equipment utilized and thedesired resultant structure. It is also to be noted that depositions maybe accomplished to fabricate the cable of the present invention usingthermal evaporation equipment and techniques, and that the presentinvention Is not therefore limited to cables produced by any particulartype of equipment, such as sputtering systems.

In FIG. 3, as mentioned previously, the ultrasonic bonding tool 44 isshown in position ready to bond the conductor 52 of the thin film cable20 to a circuit pad 54 on the detector 12. A flat surface 56 of the tip42 of the bonding tool 44 is placed against an upper surface 58 of thepolyimide substrate 46, and further positioned so that the flat surface56 of the tip 42 of the bonding tool 44 will be disposed over the goldconductor 52 in contact with the circuit pad 54 of the detector 12.

To attach the thin film cable 20 conductor 52 to the circuit pad 54,bonding energy in the form of an applied downward force in the range of15 grams-150 grams, and possibly up to 250 grams, for a time of between20 ms and 500 ms, and possibly up to two seconds, is applied to thebonding tool 44 and transferred to the surface 56 of the bonding tooltip 42. The downward pressure is calculated to cause a compression ofthe polyimide substrate 46 beyond its elastic limit. Presently it ispreferred to use a force of 65 grams for about 100 ms in order to attacha thin film cable 20 using a substrate having a thickness of 0.001 inch.The actual forces and times most advantageously used will vary dependingupon a great many circumstances. For any particular thin film cable 20,it will be necessary to increase the force and/or time if no bond isachieved and to decrease the force and/or time if unacceptable damage,such as a broken conductor 52 results.

While the bonding tool tip 42 is in pressurized contact with the uppersurface 58 of the substrate 46, ultrasonic energy in the form of a50,000 KHZ to 100,000 KHZ mechanical scrubbing signal is applied to thebonding tool tip 42 during the time that downward pressure is beingapplied to the bonding tool 44. The ultrasonic energy is transferredthrough the polyimide substrate 46 to the conductor 52 and the circuitpad 54, thereby producing a metal to metal bond between the conductor 52and the circuit pad 54. The displacement of such a scrub is preferablyon the order of 0.0001 inch. By transferring the ultrasonic energythrough the polyimide substrate 46, the need to remove a portion of thesubstrate 46, usually by toxic agents or the like, so that the bondingtool tip 42 can make actual contact with the conductor 52, as isrequired with Tape Automated Bonding, is eliminated. This simplifiesgreatly the attachment method by reducing the number of steps in theprocess and the costs and inconvenience associated with removing aportion of the polyimide. The necessity of using frail wirebonds to makethe electrical connection with the circuit pad 54 is also eliminated, asis the need for using adhesives or the like to bond the cable to asurface near the circuit pad 54 when wirebonding is used.

The bonding energy will further include an additional subsonic scrub ofa frequency of 10 HZ to 500 HZ for thicker thin film cable structures.Where necessary, the attachment area of the conductors will include alocalized reduced thickness achieved by either wet or dry processing toinsure effective energy transfer for the attachment bonding.

FIG. 4 is an illustration of an enlarged portion of the bonding tool tip42 more fully showing a first semi-circular groove 60 having a radius ofapproximately 0.0005 inch, recessed to a depth of approximately 0.0003inch, within the surface 56 of the bonding tool tip 42, and runninglongitudinally therethrough. The tip surface 56, which will have atypical diameter of 0.005 inch, will further have a second semi-circulargroove, represented in FIG. 4 by dashed line 62, having a radius ofapproximately 0.001 inch and recessed to a depth of approximately0.00075 inch, also running longitudinally therethrough but at a rightangle to the first groove 60. The first groove 60 is further disposed sothat its longitudinal axis is at a right angle to the axis of theultrasonic scrub, represented in FIG. 4 as directional line 64. Thesecond groove 62, having its longitudinal axis at a right angle to thefirst groove 60, will therefore have its longitudinal axis runningparallel to the axis 64 of ultrasonic scrubbing. The second groove isillustrated more clearly in FIG. 5, wherein the bonding tool tip 42 hasbeen rotated 90°. The relationship between the first groove 60 andsecond groove 62 is further illustrated in FIG. 6.

During the bonding process, the first groove 60 operates to facilitate agripping onto the polyimide substrate 46 by the bonding tool tip surface56, thereby allowing a more effective transfer of ultrasonic energythrough the polyimide substrate 46. The second groove 62 furtheroperates to limit the deformation of the substrate 46 and the conductor52 in the area defined by the bonding tool tip surface 56, and is largerthan the first groove 60, thus reducing the chance of a break in theconductor 52 after the attachment bond is completed.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present inventions can beimplemented in a variety of forms. Therefore, while these inventionshave been described in connection with particular examples thereof, thetrue scope of the inventions should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

What is claimed is:
 1. A method for bonding a flexible cable to anelectrical component, the component having a connection point, themethod comprisinga) providing a flexible cable having a polyimide filmsubstrate, the substrate having an upper and a lower side, the lowerside further having a layer of material affixed thereto and a pluralityof conductors deposited onto the material; b) placing the cable over thecomponent so that the end of a conductor lies on a connection point; c)pressing the upper side of the substrate over said end of a conductorwith an ultrasonic bonding tool with a pressure that compresses thepolyimide film substrate beyond its elastic limit; d) applyingultrasonic energy to the tool and through the substrate while applyingdownward pressure for a sufficient period of time to bond the conductorto the connection point; and e) ceasing pressing the upper surface ofthe substrate and the application of ultrasonic energy to the bondingtool.
 2. The method of claim 1 wherein the substrate is a polyimide filmhaving a thickness between approximately 0.0003 to 0.020 inch.
 3. Themethod of claim 2 wherein the substrate is a polyimide film having athickness between approximately 0.0003 and 0.003 inch.
 4. The method ofclaim 1 wherein the material is metal and selected from the groupcomprising (a) titanium; (b) tungsten; and (c) molybdenum and chromium.5. The method of claim 1 wherein the material is titanium having athickness between approximately 400 and 2,000 angstroms.
 6. The methodof claim 1 wherein the bonding tool comprises:a) a conically shaped tip,the tip further having a flat surface; b) a first semi-circular groovein the surface, the groove running across the entire surface of the tipand being operable to facilitate gripping onto the substrate by the tipwhen the tip is pressing against the upper side of the substrate; and c)a second semi-circular groove in the surface, the groove running acrossthe entire surface of the tip and further being at a right angle to thefirst groove, whereby the second groove operates to limit thedeformation of the substrate when the tip is pressing against the upperside of the substrate.
 7. The method of claim 1 wherein the downwardpressure applied by the bonding tool is in the range of approximately 15grams-250 grams.
 8. The method of claim 1 wherein the downward pressureis applied for a time In the range of approximately 20 milliseconds-2seconds.
 9. A method for bonding a flexible, thin, electrical cable to acircuit connection comprising:a) providing a flexible electrical cablehaving a polyimide film substrate, the substrate further having an upperand lower surface, a layer of adhesive facilitating material depositedonto the lower surface of the polyimide and at least one gold conductortrace deposited onto the adhesive facilitating material; b) placing thecable such that the lower surface of an end of the cable is in contactwith the circuit connection where a gold conductor trace on saidmaterial is deposited; c) placing a tip of an ultrasonic bonding tool incontact with the upper surface of the polyimide such that pressure onthe bonding tool exerts a force on the gold conductor trace in contactwith the circuit connection; d) pressing the tip of the bonding toolwith a force of between approximately 15 grams and 250 grams andcompressing the polyimide beyond its elastic limit for a period ofbetween approximately 20 milliseconds and two seconds; e) while applyingthe force, further applying an ultrasonic scrubbing force to the tip ofthe bonding tool, said scrubbing force at the rate of betweenapproximately 50,000 KHZ and 100,000 KHZ, to ultrasonically scrub thetip of the bonding tool, whereby ultrasonic energy is transmittedthrough the polyimide to produce a metallic bond between the goldconductor trace and the circuit connection; and f) ceasing the downwardforce and the ultrasonic energy applied to the bonding tool.
 10. Themethod of claim 9 wherein the adhesive facilitating material is titaniumdeposited onto the substrate with a thickness of approximately 400 to2,000 angstroms.